Field of the Invention
This invention relates generally to a polymerizable composition and to the polymerizate, which are particularly well suited for optical articles.
Technical Considerations
Allyl carbonate monomer compositions can be polymerized to use as transparent coatings, optical lenses, and other optical elements. Diethylene glycol bis(allyl carbonate) monomer compositions have been produced by reacting diethylene glycol with phosgene at temperatures between 0° C. and 20° C. to form the corresponding dichloroformate, which is then reacted with allyl alcohol in the presence of a suitable acid acceptor, for example, pyridine, a tertiary amine, or an alkaline or alkaline earth metal hydroxide. Alternatively, the allyl alcohol is reacted with phosgene. The resulting chloroformate is reacted with diethylene glycol in the presence of an alkaline reagent.
One problem associated with the polymerization of the known diethylene glycol bis(allyl carbonate) monomer compositions is the relatively high shrinkage of the polymerized material, which occurs during the course of polymerization to the final thermoset polymer. For example, there can be a shrinkage of greater than 13 percent during the polymerization of a conventional diethylene glycol bis(allyl carbonate) monomer. Shrinkage to such a degree is particularly detrimental in casting operations, such as those used to produce ophthalmic lenses and ophthalmic lens blanks, where the liquid monomer composition is introduced to a mold and then polymerized to the final thermoset polymer. This shrinkage can result in commercially unacceptable products that must be rejected, decreasing the yield of the process.
It is known that shrinkage in the mold may be reduced by introducing a liquid prepolymer into the mold and then polymerizing the prepolymer to the final thermoset polymer. In this known system, the prepolymer is produced by partially polymerizing the diethylene glycol bis(allyl carbonate) monomer composition to consume a portion of the polymerizable ethylenic double bonds.
Low shrinkage in the mold is favored by utilizing a large number of polymerizable ethylenic double bonds during partial polymerization to produce the prepolymer, which is introduced into the mold. However, during the partial polymerization of the diethylene glycol bis(allyl carbonate) monomer compositions, the viscosity increases as polymerizable ethylenic double bonds are consumed. For practical purposes, the viscosity should not become so high that the resulting prepolymer does not flow reasonably readily into the mold. Viscosity of the prepolymer, therefore, provides a practical lower limit to shrinkage in the mold. Production of the prepolymer can also cause processing difficulties, such as potential gelling of the reactor. It is a disadvantage to pre-polymerize the double bond structure. Production costs can be reduced and numerous problems can be avoided by eliminating the pre-thickening step.
Another challenge with conventional low shrinkage compositions is that they typically have increased length between cross-links, which produces soft polymers.
Therefore, it would be desirable to provide a reduced shrinkage composition that optimizes or improves the balance between the amount of polymerization shrinkage reduction and the final polymer's properties, for example, such that the final polymer is rigid enough to be used in various applications, such as conventional optical applications.